In addition to the systems which, through the action of UV, cure and are polymerized by free radicals, more recently cationically curable systems have been developed which contain epoxy or vinyl ether compounds. In the latter type of systems, the curing or hardening is initiated particularly by diaryliodonium and triarylsulfonium salts. There is a definite advantage in such cationically curable systems in that the curing reaction is not affected by and is insensitive to the oxygen in the air. Moreover, more rapid film hardening is accomplished and the systems are particularly harmless to the environment.
Cationically curable systems on the basis of epoxy functional organopolysiloxanes have been described in numerous patents, of which U.S. Pat. No. 4,421,904 may be mentioned as an example. A relatively comprehensive representation appears in the journal Adhesives Age, 1989, 24-27. This publication discloses and describes epoxy group containing siloxanes as they are available on the market for the abhesive coating of flat carrier material.
With a view to accomplishing cationic polymerization of compounds which contain vinyl ether groups, which may be substituted, catalysts of low acid strength are sufficient. Contrary to compounds containing epoxy groups, not only the customary salt-like diaryliodonium and triarylsulfonium compounds, but also non-salt-like compounds such as, for example, ketosulfones may be used as photo initiators.
Cationically curable vinyl ether groups containing organopolysiloxanes have been disclosed in U.S. Pat. No. 4,617,238. This patent discloses a procedure wherein the vinyl ether group is introduced into the organopolysiloxane by addition (hydrosilylation) of compounds having an allyl and a vinyl ether group to an SiH group of the organopolysiloxane. In doing so, only the allyl groups is added to the SiH group while the vinyl ether group is maintained. It follows from the above that per SiH group, in each case only one vinyl ether group can be incorporated into the siloxane molecule.